1. Field of the Invention
The invention relates in general to a digital signal broadcasting technology, and more particularly, to a technology for estimating a channel effect of a multipath environment.
2. Description of the Related Art
Developments of digital television broadcasting have gradually matured with progresses of communication technologies. Besides being transmitted by cables, digital television signals may also be transmitted in form of wireless signals via base stations or artificial satellites. Digital Video Broadcasting-Terrestrial (DVB-T) and Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) are prevalent standards adopted in the field of digital television broadcasting.
Multipath exists in most wireless communication environments. A receiver needs to evaluate a corresponding channel effect in order to correctly extract and utilize received data, e.g., to correctly identify boundaries between symbols in the signals. Taking DVB-T and ISDB-T signals for example, a possible outcome resulted from a channel effect misjudged by a receiver is given below.
FIG. 1(A) shows an example of a signal compliant to the DVB-T and ISDB-T specifications. The beginning of each symbol includes a section of channel pilot. The channel pilot is a duplication of an ending section of the symbol it belongs. That is to say, contents of the section CP1 are identical to those of the section E1, and contents of the section CP2 are identical to those of the section E2. FIG. 1(B) shows a possible result of the signal arriving at the receiver after passing through a multipath environment. At the time point t1, the signal transmitted via the first path (to be referred to as the first signal) first reaches the receiver; whereas the same signal transmitted via the second path (to be referred to as the second signal) only reaches the receiver at the time point t2. To identify a boundary between symbols, a receiver usually performs correlation calculation on received signals. The size of two sampling windows for the correlation calculation is the length of the section of the channel pilot, and a gap between the two sampling windows is fixed as a distance between the channel pilot section and the corresponding ending section. As seen from FIG. 1(C), the correlation is the highest when the two selected sampling windows (represented by the shaded areas) slide to the section CP1 and the section E1. In comparison, the correlation is lower when the two sampling windows are relocated to positions depicted in FIG. 1(D).
When considering the first signal alone, the relationship between its correlation result and the time is as the curve CR1 shown in FIG. 1(E). When considering the second signal alone, the relationship between its correlation result and the time is as the curve CR2 (assuming that the strength of the second signal is weaker, the peak of CR2 is lower than that of CR1). In practice, however, rather than independent first and second signals, the signal received at the receiver is a summed result (to be referred to as a summed signal) of the first signal and the second signal. Therefore, the curve representing the correlation result obtained by the receiver is the curve CR in FIG. 1(E), which is a sum of the curves CR1 and CR2.
As seen from FIG. 1(E), boundaries between the symbols in the first signal can be easily identified according to positions of the peaks of the curve CR1. Similarly, according to positions of the peaks of the curve CR2, boundaries between the symbols in the second signal can also be clearly identified. However, according to the curve CR, boundaries between the symbols in the summed signal cannot be directly and easily determined. This determination task is made even more challenging as a multipath environment that the summed signal passes through gets more complex or the level of signal interference imposed on the summed signal during the transmission process gets higher. Given that the receiver identifies the channel effect of the multipath environment that the summed signal passes through, parts accounted by the curves CR1 and CR2 can be identified from the curve CR, so that the first signal and the second signal can be retrieved from the summed signal and more ideal symbol boundaries can be selected. Without accurate evaluation on the channel effect of the multipath environment, symbol boundaries are likely misjudged at the receiver to further result in performance degradation in the reception system.
As shown in FIG. 2, in many wireless communication systems, the signal received by the receiver at a single time point tx simultaneously contains various frequency components (F1 to FN), i.e., the reception signal contains contents carried by numerous different types of subcarriers. The channel effect of the multipath environment that the signal passes through is in fact the sum of respective frequency-domain channel effects H1 to HN corresponding to the subcarriers F1 to FN. That is to say, the most ideal approach for accurately estimating the channel effect of the multipath environment of the signal is to identify H1 to HN. However, because the data contents carried by the subcarriers maybe unknown to the receiver and a large amount of time is required for identifying all of the frequency-domain channel effects H1 to HN, such approach is not usually employed by the receiver. In practice, a possible approach is to estimate the frequency-domain channel effects corresponding to certain subcarriers; for example, frequency-domain channel effects (H0, H3, H6, H9 . . . ) corresponding to subcarriers (F0, F3, F6, F9 . . . ) having frequency indices in multiples of 3 are estimated. The receiver may then perform inverse fast Fourier transform (IFFT) on the channel effects to identify the corresponding time-domain channel effects.
As shown in FIG. 3(A), since each of the subcarriers (F0, F3, F6, F9 . . . ) having frequency indices in multiples of 3 carries a scatter pilot at a predetermined interval, the receiver only utilizes the frequency-domain channel effects corresponding to frequency indices in multiples of 3 as sampling values, and thus the IFFT results include three time-domain channel effects. Among the three time-domain channel effects, one is the real time-domain channel effect corresponding to the multipath environment, whereas the other two are duplications of the time-domain channel effect. In this example, the real time-domain channel effect of the multipath environment is possibly h1 in FIG. 3(B), or h2 in FIG. 3(C). The receiver needs to select either h1 or h2 as the time-domain channel effect to represent the multipath environment.
In the prior art, the receiver usually performs extensive tests on reception signals using h1 and h2, and selects h1 or h2 as the time-domain channel effect according to the test results. For example, a typical DVB-T receiver may determine respective corresponding symbol boundaries according to h1 and h2, and continuously analyze a bit error rate (BER) of multiple symbols according to the two different symbol boundaries. The receiver then selects the time-domain channel effect corresponding to the lower BER. One of the setbacks of such solution that selects one channel effect from a plurality of channel effects by carrying out extensive tests is extremely time-consuming. For a television system, this setback causes a long waiting period before a user can observe a correct image on the screen when switching channels.